1. Field of the Invention
This invention relates to acoustic thermography. More specifically, the invention is a system and method for air-coupled acoustic thermography that can be used for in-situ evaluation of structures.
2. Description of the Related Art
Aircraft structures are increasingly being made from advanced composite sandwich structures typically defined by a lightweight core (e.g., a honeycomb) faced with thin composite skins. These structures are relatively inexpensive, lightweight, repairable, and can be molded into complex shapes. From time-to-time, aircraft incorporating these composite sandwich structures must be inspected for damage In particular, the skins or facing must be inspected, the bond between the skin/facings and the core must be inspected, and the core itself must be inspected. Since these structures are prone to moisture absorption through minute skin imperfections or damages, the inspection system must be sensitive. An effective inspection regimen and/or system must not only perform all of these inspections with a high-degree of sensitivity, but must do so in a non-destructive fashion while being performable “in the field” for a variety of types of composite sandwich structures.
A variety of known inspection regimens/testing systems have been used to inspect composite sandwich structures. Unfortunately, current approaches suffer from one or more drawbacks. For example, conventional ultrasonic inspection systems require direct contact with the structure and/or the use of a couplant (e.g., gel, water, etc.). Shearography illuminates a structure's surface with light and detects speckle interference patterns while a load (e.g., a vacuum) is applied to the structure's surface. However, it is difficult to determine defect size/shape through interpretation of fringe pattern data produced by this approach.
More recently, acoustic thermography has been used for flaw/damage detection. For example, U.S. Pat. No. 7,060,971 discloses a flaw inspection system that transmits ultrasonic waves towards a structure in order to generate a thermal signature that can be read by an infrared camera. Flaws or damages show up as “hot spots.” However, reference blocks having known flaws/defects must be attached to the structure's surface in the area being inspected in order to assure that the proper amount of acoustic energy is being used. The continual need to attach/detach the reference blocks adds time and cost to the inspection regimen, poses risks of damage to the structure's surface, and presents problems for inspection repeatability.
Another acoustic inspection system method is disclosed in U.S. Pat. No. 7,297,952. A broadband acoustic signal is introduced into a structure where the signal includes at least two non-harmonically related frequencies. An infrared camera is used to capture both a background image and a series of images of the structure after the acoustic signal application. Correlations between the background image and series of images are performed on a pixel-by-pixel basis. However, this system is not sensitive enough for inspection of composite sandwich structures.